feat(rules): forbid repeating a word already on the board in Erudit

Russian "Эрудит" treats a word laid on the board as belonging to the
game: it cannot be laid again. Neither the solver, the backend nor the
offline JS port knew the rule, so a player (and the robot) could replay a
word freely. Official Scrabble places no such restriction, so both
Scrabble variants keep playing unrestricted.

The rule applies in two ways. A play whose main word is already on the
board is illegal, and is neither accepted nor generated. A play whose
perpendicular cross-word is already there stands — that word is
incidental to laying the main word — but scores nothing. The set of
played words is the game's own move journal, main words and cross-words
alike, compared decoded, so a word spelled with a blank is the same word.

It lives in the game layer, not the solver: only a game knows its
history, and the solver stays stateless and standard-rules. The backend
applies it at submit, at the move preview and over generated moves
(filtering and re-ranking them, so neither the robot nor the hint can
offer a play the engine would then refuse); the client port does the same
for the offline engine and for the on-device preview of an online game.

The rule is pinned per game (games.no_repeat_words, set from the variant
at creation) rather than keyed on the variant, because a game is replayed
from its journal on every open. Applied retroactively it would make an
already-played repeat illegal — closing that game as a draw — and would
rescore a play whose cross-word repeats an earlier word, shifting a live
game's totals. Games created before the rule keep playing without it. The
flag rides the wire as a trailing field because the client's preview must
score the way the server does, and offline games pin the same answer in
their own record.

backend/internal/postgres/jet/backend/model/games.go

@@ -34,4 +34,5 @@ type Games struct {
 	MultipleWordsPerTurn bool
 	VsAi                 bool
 	GameKind             int16
+	NoRepeatWords        bool
 }

backend/internal/postgres/jet/backend/table/games.go

@@ -38,6 +38,7 @@ type gamesTable struct {
 	MultipleWordsPerTurn postgres.ColumnBool
 	VsAi                 postgres.ColumnBool
 	GameKind             postgres.ColumnInteger
+	NoRepeatWords        postgres.ColumnBool
 
 	AllColumns     postgres.ColumnList
 	MutableColumns postgres.ColumnList
@@ -100,9 +101,10 @@ func newGamesTableImpl(schemaName, tableName, alias string) gamesTable {
 		MultipleWordsPerTurnColumn = postgres.BoolColumn("multiple_words_per_turn")
 		VsAiColumn                 = postgres.BoolColumn("vs_ai")
 		GameKindColumn             = postgres.IntegerColumn("game_kind")
-		allColumns                 = postgres.ColumnList{GameIDColumn, VariantColumn, DictVersionColumn, SeedColumn, StatusColumn, PlayersColumn, ToMoveColumn, TurnStartedAtColumn, TurnTimeoutSecsColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, EndReasonColumn, CreatedAtColumn, UpdatedAtColumn, FinishedAtColumn, OpenDeadlineAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn}
-		mutableColumns             = postgres.ColumnList{VariantColumn, DictVersionColumn, SeedColumn, StatusColumn, PlayersColumn, ToMoveColumn, TurnStartedAtColumn, TurnTimeoutSecsColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, EndReasonColumn, CreatedAtColumn, UpdatedAtColumn, FinishedAtColumn, OpenDeadlineAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn}
-		defaultColumns             = postgres.ColumnList{StatusColumn, ToMoveColumn, TurnStartedAtColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, CreatedAtColumn, UpdatedAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn}
+		NoRepeatWordsColumn        = postgres.BoolColumn("no_repeat_words")
+		allColumns                 = postgres.ColumnList{GameIDColumn, VariantColumn, DictVersionColumn, SeedColumn, StatusColumn, PlayersColumn, ToMoveColumn, TurnStartedAtColumn, TurnTimeoutSecsColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, EndReasonColumn, CreatedAtColumn, UpdatedAtColumn, FinishedAtColumn, OpenDeadlineAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn, NoRepeatWordsColumn}
+		mutableColumns             = postgres.ColumnList{VariantColumn, DictVersionColumn, SeedColumn, StatusColumn, PlayersColumn, ToMoveColumn, TurnStartedAtColumn, TurnTimeoutSecsColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, EndReasonColumn, CreatedAtColumn, UpdatedAtColumn, FinishedAtColumn, OpenDeadlineAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn, NoRepeatWordsColumn}
+		defaultColumns             = postgres.ColumnList{StatusColumn, ToMoveColumn, TurnStartedAtColumn, HintsAllowedColumn, HintsPerPlayerColumn, MoveCountColumn, CreatedAtColumn, UpdatedAtColumn, DropoutTilesColumn, MultipleWordsPerTurnColumn, VsAiColumn, GameKindColumn, NoRepeatWordsColumn}
 	)
 
 	return gamesTable{
@@ -130,6 +132,7 @@ func newGamesTableImpl(schemaName, tableName, alias string) gamesTable {
 		MultipleWordsPerTurn: MultipleWordsPerTurnColumn,
 		VsAi:                 VsAiColumn,
 		GameKind:             GameKindColumn,
+		NoRepeatWords:        NoRepeatWordsColumn,
 
 		AllColumns:     allColumns,
 		MutableColumns: mutableColumns,

backend/internal/postgres/migrations/00017_no_repeat_words.sql

@@ -0,0 +1,14 @@
+-- The "Эрудит" no-repeat-words rule: a word laid on the board cannot be laid again. It is pinned
+-- per game rather than derived from the variant, because a game is replayed from its journal on
+-- every open — a rule applied retroactively would make an already-played repeat illegal (voiding
+-- the game) and would rescore a play whose cross-word repeats an earlier word. Every existing game
+-- therefore keeps the unrestricted rule (DEFAULT false, no backfill) and only new erudit_ru games
+-- are created with it on. Additive column — applies forward via goose with no data rewrite (no
+-- contour wipe); an image rollback ignores the column.
+-- +goose Up
+
+ALTER TABLE backend.games ADD COLUMN no_repeat_words boolean NOT NULL DEFAULT false;
+
+-- +goose Down
+
+ALTER TABLE backend.games DROP COLUMN no_repeat_words;